Adsorption dryers of this kind are well-known. As far as they comprise two or more pressure vessels, these usually operate alternately between “drying and regeneration”. With such adsorption dryers known from practice the pressure vessel consists of a cylindrical tube with pistons installed therein, sealed on both sides of the drying agent. The pistons on the side turned away from the drying agent are formed integrally with screw-type plugs, which can be screwed into the tube. The production and the assembly of such pistons comprising screwed plugs are complex. Furthermore, the screw connection is sensitive and therefore prone to problems.